Kinetics of the Reactions of NO3 Radical with Methacrylate Esters

Atmospheric oxidation capacity and secondary pollutant formation potentials based on photochemical loss of VOCs in a megacity of the Sichuan Basin, China

Volatile organic compounds (VOCs) are significant precursors to photochemical pollution. However, reactive VOC species are easily oxidized during transportation, resulting in a systematic underestimate of the measured concentrations. To address this, we applied an improved calculation method to correct the measured VOC concentrations into photochemical initial concentrations (PICs) in Chengdu, a megacity in the Sichuan Basin, China, which is highly vulnerable to complex pollution. In this study, 56 VOC species on the Photochemical Assessment Monitor Station (PAMS) target list were quantitatively monitored throughout all four seasons. Comparing to directly measured values, photochemically initialized total mixing ratios of VOCs increased by 18.6 % in general. The photochemical loss percentages of alkenes and aromatics were prominent in summer (68.6 %, 28.7 %) and spring (65.9 %, 24.7 %), respectively. Furthermore, we examined contributions of VOCs to atmospheric oxidation capacity (AOC) depending on PICs and found that maximum daily total AOC showed a surge in spring and summer. Besides hydroxyl radicals, daytime O3 in spring and late-afternoon nitrate radicals in summer were essential for AOC with PICs. As expected, alkenes and aromatics dominated PIC-based ozone formation potentials (OFPs). Furthermore, contribution of alkenes to secondary organic aerosol formation potentials reached 15.5 % and 7.6 % in spring and summer, respectively. Using positive matrix factorization model, we identified five VOC sources including vehicular exhaust, industrial emissions, solvent usage, biogenic sources, and liquefied petroleum gas/natural gas use. Based on PICs, biogenic sources were significantly underestimated in spring and summer. Meanwhile, m,p-xylene from solvent usage and isoprene from biogenic sources were the primary contributors to OFPs. Consequently, these results emphasize the significance of photochemically oxidized VOC concentrations, especially for reactive species in typical seasons.

Important Oxidants and Their Impact on the Environmental Effects of Aerosols

Oxidants are central species in the atmosphere, where they not only determine secondary particle formation but also impact human health and climate change. In general, they are unstable, highly reactive, and recyclable and have been studied in field observations, laboratory studies, and model simulations. The most widely investigated oxidants, such as OH radicals, O₃, and Cl atom, HONO, NO₃, N₂O₅, and Criegee Intermediates (CIs) have attracted more attention recently. Furthermore, secondary particles formed in the oxidations processes impact the particle physicochemical properties, such as hygroscopicity and optical properties and therefore impact the atmospheric radiation balance. Therefore, the newest investigation results of important oxidants (HONO, NO₃, N₂O₅, and CIs) are reviewed in this manuscript, and the environmental effects of secondary particles formed through corresponding oxidation processes are also stated. Furthermore, some perspectives are further discussed in the article.

Kinetic and product studies of the reactions of NO3 with a series of unsaturated organic compounds

Rate coefficients for the reaction of NO₃ radicals with 6 unsaturated volatile organic compounds (VOCs) in a 7300 L simulation chamber at ambient temperature and pressure have been determined by the relative rate method. The resulting rate coefficients were determined for isoprene, 2-carene, 3-carene, methyl vinyl ketone (MVK), methacrolein (MACR) and crotonaldehyde (CA), as (6.6 ± 0.8) × 10^-13, (1.8 ± 0.6) × 10^-11, (8.7 ± 0.5) × 10^-12, (1.24 ± 1.04) × 10^-16, (3.3 ± 0.9) × 10^-15 and (5.7 ± 1.2) × 10^-15 cm³/(molecule•sec), respectively. The experiments indicate that NO₃ radical reactions with all the studied unsaturated VOCs proceed through addition to the olefinic bond, however, it indicates that the introduction of a carbonyl group into unsaturated VOCs can deactivate the neighboring olefinic bond towards reaction with the NO₃ radical, which is to be expected since the presence of these electron-withdrawing substituents will reduce the electron density in the π orbitals of the alkenes, and will therefore reduce the rate coefficient of these electrophilic addition reactions. In addition, we investigated the product formation from the reactions of 2-carene and 3-carene with the NO₃ radical. Qualitative identification of an epoxide (C₁₀H₁₆OH⁺), caronaldehyde (C₁₀H₁₆O₂H⁺) and nitrooxy-ketone (C₁₀H₁₆O₄NH⁺) was achieved using a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and a reaction mechanism is proposed.

A Combined Experimental and Theoretical Study on the Gas Phase Reaction of OH Radicals with Ethyl Propyl Ether

The reaction of ethyl propyl ether (EnPE) with OH radicals was studied using proton-transfer-reaction mass spectrum (PTR-MS), and the rate constant was measured at 298 K and atmospheric pressure using the relative rate method: k_exp(OH+EnPE) = (1.13 ± 0.09) × 10^-11 cm³ molecules^-1 s^-1. In addition, a parallel theoretical study was performed using the traditional transition state theory (TST) with a tunnelling effect correction in combination at M05-2X method with two basis sets, 6-311++G(d,p) and aug-cc-pVTZ. According to these calculations, H atom abstraction occurs more favorably from the methylene group adjacent to the -O- bond than from the other groups. The theoretical calculation of the total rate constant of the reaction of EnPE with OH radicals was consistent with the experimental values. The gas-phase products indicated that the major products observed were ethyl formate, ethyl propionate, propionic acid. Combined with the experimental and theoretical results, the possible reaction mechanisms were proposed and discussed. The atmospheric implications of the studied reaction are presented, and the lifetime of EnPE in the presence of OH radicals was evaluated to be approximately 1 day.

Kinetics of the reactions of NO3 radical with alkanes

The rate coefficients for the reactions of NO₃ radicals with methane (CH₄), ethane (C₂H₆), propane (C₃H₈), n-butane (n-C₄H₁₀), iso-butane (iso-C₄H₁₀), 2,3-dimethylbutane (C₆H₁₄), cyclopentane (C₅H₁₀) and cyclohexane (C₆H₁₂) at atmosphere pressure (1000 ± 5 hPa) and room temperature (298 ± 1.5 K) were measured using an absolute method.

Atmospheric loss of nitrous oxide (N2O) is not influenced by its potential reactions with OH and NO3radicals

The rate coefficient for the possible reaction of OH and NO₃radical with N₂O are shown to be, respectively, <1 × 10⁻¹⁷and <5 × 10⁻²⁰cm³molecule⁻¹s⁻¹. They are too low to contribute significantly to the atmospheric removal of N₂O.

A quantum theory investigation on atmospheric oxidation mechanisms of acrylic acid by OH radical and its implication for atmospheric chemistry

The hydroxyl radical, as the most important oxidant, controls the removal of some volatile organic compounds (VOCs) in the atmosphere. In this work, the atmospheric oxidation processes of acrylic acid by OH radical have been investigated by density functional theory (DFT). The energetic routes of the reaction of CH₂CHCOOH with OH radical have been calculated accurately at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G(d,p) level. It is implicated that the oxidation has five elementary reaction pathways mostly hinging on how hydroxyl radical approaches to the carbon skeleton of acrylic acid. The atmospheric degradation mechanisms of the CH₂CHCOOH by OH radical are the formation of reactive intermediates IM1 and IM2. Meanwhile, the further oxidation mechanisms of IM1 and IM2 by O₃ and NO are also investigated. The rate coefficients have been computed using tight transition state theory of the variflex code. The calculated rate coefficient is 2.3 × 10^-11 cm³ molecule^-1 s^-1 at standard pressure and 298 K, which is very close to the laboratory data (1.75 ± 0.47 × 10^-11 cm³ molecule^-1 s^-1). Moreover, the atmospheric lifetime of acrylic acid is about 6 h at 298 K and 1 atm, implying that the fast sinks of acrylic acid by hydroxyl radical.

Kinetics of atmospheric reactions of 4-chloro-1-butene

Chloroalkenes are among the important anthropogenic organic compounds emitted in the atmosphere as a result of their wide use in synthetic processes in industry. Despite their well-known adverse effects on human health and air quality, the chemistry of these chloroalkenes remains poorly explored. In this work, reactions of 4-chloro-1-butene (CBE), a representative example of chloroalkenes, with O₃, OH, NO₃, and Cl are investigated in a 100-L Teflon reaction chamber equipped with gas chromatography-flame ionization detector (GC-FID). The absolute rate method was used for the reaction with O₃ while the relative rate method was used for reactions with OH, NO₃, and Cl. The following rate constants were obtained at room temperature (298 ± 2) K and atmospheric pressure: (3.96 ± 0.43) × 10^-18, (2.63 ± 0.96) × 10^-11, (4.48 ± 1.23) × 10^-15, and (2.35 ± 0.90) × 10^-10 cm³ molecule^-1 s^-1, for reactions with O₃, OH, NO₃, and Cl, respectively. Atmospheric lifetimes of CBE calculated from rate constants of the different reactions obtained in this work showed that reaction with OH is the main loss process for CBE, while in coastal areas and in the marine boundary layer, the CBE loss by Cl reaction becomes important. Estimation of the value of the photochemical ozone creation potential (POCP) indicated that CBE has a large ozone formation potential. The present work underlines the need for further studies on the atmospheric chemistry of chlorinated VOCs.

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl, and NO3: Reaction Kinetics and Mechanism

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O₃, OH, Cl, and NO₃) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm³ molecule^-1 s^-1) are (1.8 ± 0.3) × 10^-18, (3.1 ± 0.7) × 10^-11, (2.5 ± 0.5) × 10^-10, and (1.1 ± 0.4) × 10^-14, for reactions with O₃, OH, Cl, and NO_3, respectively. While results for reactions with O₃, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO₃ is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τ_O3 = 9 days, τ_OH = 5 h, τ_Cl = 5 days, τ_NO3 = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime.